Bad Air Days in Guilford County, North Carolina: 1 a Year

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 655 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (36% particle days).

The bad-air season here

Most bad days here fall in May and June and July. Averaged over 2016 to 2025.

What to buy, and what not to

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The record for Guilford

In 2000 to 2009 this county averaged 24 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2016, with 4 bad days on 366 monitored days.

Date	AQI	Main pollutant
2016-05-26	133	Ozone
2023-06-29	131	PM2.5
2023-04-20	122	Ozone
2024-08-26	122	Ozone
2016-06-10	119	Ozone

The air on a normal day

Across 3,642 monitored days from 2016 to 2025, the air in Guilford County rated Good on 65% of them, less often than the 72% typical of monitored counties, and Moderate on 34%. The median day sat at AQI 45; one day in ten went above 61.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)59%2,132 days
ozone39%1,422 days
coarse dust (PM10)2%88 days

Guilford County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	274	91	0	0	41	77
2024	366	249	116	1	0	44	122
2023	365	183	178	4	0	50	131
2022	365	223	142	0	0	46	84
2021	365	245	120	0	0	44	100
2020	365	288	77	0	0	40	88
2019	365	244	121	0	0	44	90
2018	355	238	116	1	0	44	105
2017	365	229	135	1	0	45	112
2016	366	210	152	4	0	48	133
2015	364	216	147	1	0	46	112
2014	353	182	171	0	0	50	100
2013	357	223	133	1	0	44	105
2012	364	173	183	8	1	51	154
2011	361	154	196	11	0	53	140
2010	364	136	209	19	1	54	154

*2025 may be a partial year in EPA's file.

North Carolina compared

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.